Homozygous familial hypercholesterolemia (FH) is an inherited disease characterized by massive elevations in serum cholesterol, premature and accelerated atherosclerosis, and early cardiac death. FH arises from alleles encoding defective low density lipoprotein (LDL) receptors. Liver-directed gene therapy is being used to effect a metabolic cure by reconstituting hepatic expression of normal LDL receptors. Clinical trials of ex vivo gene therapy for FH are currently underway at our institution, under the auspices of Dr. James Wilson and the Institute for Human Gene Therapy (IHGT). The Radiology Department and IHGT have formed a collaboration to develop methods for the noninvasive detection of transgene products in vivo. In particular, we are working to develop MR contrast agents (and, more recently, radiopharmaceuticals) targeted to the human LDL receptor transgene product. Our first strategy was to prepare LDL particles reconstituted with hydrophobic superparamagnetic iron oxide (SPIO) nanoparticles. Twenty different types of reconstituted LDL particles were prepared using a variety of apolar solvents, SPIO surfactants, and apoprotein-B100 (apo-B) stabilizing agents. We are now pursuing a different strategy for preparing these contrast agents. Surfactant-stabilized hydrophilic SPIO particles or multimeric gadolinium chelates will be covalently attached to apo-B or to the sugar moiety on apo-B. Preliminary studies will explore the specific kinds of covalent modification to apo-B that result in retention of receptor binding activity. LDL receptor-targeted contrast agents that show binding activity in vitro will be subjected to in vivo testing using the Watanabe Heritable Hyperlipidemic rabbit and the LDL receptor knockout mouse, two animal models of human FH.